Should `isom` brand be used to detect MP4?

[evilpie]

Canon has introduced a new RAW image format CR3 which is based on the iso mp4 container

https://gitlab.freedesktop.org/xdg/shared-mime-info/-/issues/157
https://github.com/lclevy/canon_cr3/blob/master/readme.md

This caused Firefox to automatically sniff a user's CR3 file download as an MP4 video file: https://bugzilla.mozilla.org/show_bug.cgi?id=1725190.

[GPHemsley]

I don't think we want to be sniffing the CR3 format specifically, either positively or negatively.

However, a later comment in the linked bug indicates that Firefox is using the isom brand to detect it as MP4.

Since that brand is not part of the MP4 sniffing algorithm, the question on the table is: should the MP4 sniffing algorithm be changed to include the isom brand, or should Firefox be changed to remove it?